1. Field of the Invention
The present invention relates to a semiconductor disk device having a semiconductor memory element such as a flash memory serving as a storage medium.
2. Description of the Related Art
Currently, magnetic disk storage devices such as hard disk drives and floppy disk drives are widely used as external storage devices for a computer systems. However, these magnetic disk storage devices have the disadvantage that they consume large amounts of electricity. As an alternative external storage device to the magnetic disk storage devices having such a disadvantage, semiconductor disk devices having a semiconductor memory element such as a flash memory element are receiving much attention.
FIG. 10 illustrates a configuration of a conventional semiconductor disk device of this type. In FIG. 10, the semiconductor disk device inputs and outputs data from or to an external host system via an interface circuit 3. A CPU 4 receives the data that is input via the interface circuit 3 and writes the data into a flash memory 5. The CPU 4 also reads data from the flash memory 5 and outputs the data to the host system via the interface circuit 3. The flash memory 5 stores the above received data. As described above, the conventional semiconductor disk device comprises the interface circuit 3, CPU 4, and flash memory 5.
The flash memory 5 is a non-volatile memory capable of electrically writing and erasing data. In an erasing operation, an entire data block consisting of several tens of Kbytes to several ten Kbytes is erased at one time. The CPU 4 also acts as a memory controller for controlling the flash memory 5.
As shown in FIG. 10, the semiconductor disk device 1 is connected to an external host system 11 such as a computer.
The semiconductor disk device 1 shown in FIG. 10 operates in a similar manner to that of conventional hard disk storage devices. That is, the semiconductor disk device 1 receives data from the host computer 11 and stores the received data in the flash memory 5, and reads data from the flash memory 5 according to instructions issued by the host computer 11. Unlike the conventional hard disk storage devices, however, the semiconductor disk device 1 cannot overwrite data. Therefore, it is necessary to erase the entire area of a particular block before writing data in it.
Conventional semiconductor disk devices are constructed with a flash memory in a fashion such as that shown in FIG. 10, and therefore, as described above, it is necessary to perform block-erasing before writing data. However, the data erasing time of a flash memory varies from block to block, and therefore the data writing time also varies from block to block. As a result, the characteristics and life of the flash memory vary from block to block, which sometimes causes instability in operation, and which further causes degradation of the overall performance of the semiconductor disk device.